Phase Of Mass Exchange Research Articles

Binary evolution using the theory of osculating orbits

Our aim is to calculate the evolution of Algol binaries within the framework of the osculating orbital theory, which considers the perturbing forces acting on the orbit of each star arising from mass exchange via Roche lobe overflow (RLOF). The scheme is compared to results calculated from a `classical' prescription. Using our stellar binary evolution code BINSTAR, we calculate the orbital evolution of Algol binaries undergoing case A and case B mass transfer, by applying the osculating scheme. The velocities of the ejected and accreted material are evaluated by solving the restricted three-body equations of motion, within the ballistic approximation. This allows us to determine the change of linear momentum of each star, and the gravitational force applied by the mass transfer stream. Torques applied on the stellar spins by tides and mass transfer are also considered. Using the osculating formalism gives shorter post-mass transfer orbital periods typically by a factor of 4 compared to the classical scheme, owing to the gravitational force applied onto the stars by the mass transfer stream. Additionally, during the rapid phase of mass exchange, the donor star is spun down on a timescale shorter than the tidal synchronization timescale, leading to sub-synchronous rotation. Consequently, between 15 and 20 per cent of the material leaving the inner-Lagrangian point is accreted back onto the donor (so-called `self-accretion'), further enhancing orbital shrinkage. Self-accretion, and the sink of orbital angular momentum which mass transfer provides, may potentially lead to more contact binaries. Even though Algols are mainly considered, the osculating prescription is applicable to all types of interacting binaries, including those with eccentric orbits.

Magnetic survey of emission line B‐type stars with FORS 1 at the VLT

AbstractWe report the results of our search for magnetic fields in a sample of 16 field Be stars, the binary emission‐line B‐type star υ Sgr, and in a sample of fourteen members of the open young cluster NGC3766 in the Carina spiral arm. The sample of cluster members includes Be stars, normal B‐type stars and He‐strong/He‐weak stars. Nine Be stars have been studied with magnetic field time series obtained over ∼1 hour to get an insight into the temporal behaviour and the correlation of magnetic field properties with dynamical phenomena taking place in Be star atmospheres. The spectropolarimetric data were obtained at the European Southern Observatory with the multi‐mode instrument FORS1 installed at the 8m Kueyen telescope. We detect weak photospheric magnetic fields in four field Be stars, HD 62367, μ Cen, o Aqr, and ε Tuc. The strongest longitudinal magnetic field, 〈Bz〉 = 117 ± 38 G, was detected in the Be star HD 62367. Among the Be stars studied with time series, one Be star, λ Eri, displays cyclic variability of the magnetic field with a period of 21.12 min. The binary star υ Sgr, in the initial rapid phase of mass exchange between the two components with strong emission lines in the visible spectrum, is a magnetic variable star, probably on a timescale of a few months. The maximum longitudinal magnetic field 〈Bz〉 = –102 ± 10 G at MJD 54333.018 was measured using hydrogen lines. The cluster NGC3766 seems to be extremely interesting, where we find evidence for the presence of a magnetic field in seven early B‐type stars out of the observed fourteen cluster members (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Evidence for weak magnetic fields in early‐type emission stars

AbstractWe report the results of our study of magnetic fields in a sample of 15 Be stars using spectropolarimetric data obtained at the European Southern Observatory with the multi‐mode instrument FORS 1 installed at the 8m Kueyen telescope. We detect weak photospheric magnetic fields in four stars, HD56014, HD148184, HD155806, and HD181615. We note that for HD181615 the evolutionary status is not obvious due to the fact that it is a binary system currently observed in the initial rapid phase of mass exchange between the two components. Further, we notify the possible presence of distinct circular polarisation features in the circumstellar components of Ca II H&K in three stars, HD58011, HD117357, and HD181615, hinting at a probable presence of magnetic fields in the circumstellar mass loss disks of these stars. We emphasize the need for future spectropolarimetric observations of Be stars with detected magnetic fields to study the temporal evolution of their magnetic fields and the correlation of magnetic field properties with dynamical phenomena taking place in the gaseous circumstellar disks of these stars. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Properties and nature of Be stars

Context.Binaries observed in the initial rapid phase of mass exchange between the components are very rare since the statistical probability of finding them is low. At the same time, thorough studies of them are extremely important for better understanding the process of large-scale mass exchange and possible mass loss from the system. One of these objects is probably υ Sgr.

S Equulei: A Low Mass Ratio Algol‐Type Eclipsing Binary at the Active Phase of Mass Transfer

Orbital period change of a low mass ratio (q = 0.13) Algol-type eclipsing binary, S Equulei, is presented based on the analysis of its century-long times of light minimum. Alternate period changes are discovered to superimpose on a rather rapid period increase with rate of dP/dt = +1.27 x 10(-6) days yr(-1). The alternate period changes can be explained either by the variation in the gravitational quadruple momentum via the cyclic magnetic activity of the G-type secondary or by the light-time effect via the presence of a third body. Since the third-body assumption is in good agreement with Zola's photometric solution, S Equ may be a truly triple system. The third body is rotating in an eccentric orbit (e' = 0.37) with an orbital inclination of i' - 16degrees, which is rather smaller than that of the eclipsing binary system (i = 88.degrees3). This indicates that the third body may be captured by the eclipsing pair. New accurate photometric and spectroscopic data over the next decade are very important to verify this conclusion. The rapid period increase suggests that S Equ is a low mass ratio Algol-type binary system in an active phase of mass exchange. To satisfy such a period increase, a conservative mass transfer from the less massive to the more massive components would be of the order dm/dt = 4.94 x 10(-8) M. yr(-1).

Studies of the evolutionary relationship between detached and contact binaries

We estimate the extent to which there is an evolutionary relationship between detached main-sequence binaries and ∼KW, KW, and KP contact binaries in the first mass-exchange phase. The current and initial distributions of close binaries of these types are calculated per unit volume of space in the vicinity of the Sun and used to demonstrate evolutionary transitions from low-mass, short-period, detached systems to contact binaries.

Emission Line Profiles in RY Scuti: A Massive Binary in a Pre-WR Stage?

RY Scuti (HD 169515) is a massive 9thmag eclipsing binary (Porb = 11.12d) surrounded by a peculiar nebula. High resolution spectroscopic observations in Hα, He I (λ5876), [N II] (λλ6548, 6584), [A III] (λ7136) and [S III] (λλ9069, 9532) are presented. All emission lines show a complex profile with two main structures: a sharp and strong red component and a very broad fainter complex blue one in which at least three sub-components can be detected. We have analysed these profiles in order to derive information on the velocity field in the nebula. Permitted and forbidden lines show the same velocity field indicating a common line forming region. The presence of a multi-structured blue component in all profiles gives evidence that velocity gradients are present within the nebula. Systemic corrected expansion velocities of + 30 and −45,−30,−9 Km/s are found for the red and the three blue components respectively. Weaker emission structures, reflecting the same asymmetries of the strong emissions, are observed close to the permitted lines at larger velocities (Vb = −189 Km/s and Vr = 158 K m/s). The observed velocity field indicates an asymmetric mass outflow from the system very likely through the second Lagrange point rather than via stellar wind. The system should loose mass during the mass exchange phase. Using the masses (M1 = 39M⊙, M2 = 49M⊙) and radii (R1 = 37R⊙, R2 = 41R⊙) of the two components (Milano et al., 1981), and locating RY Scuti in the [log R, log M] diagram for early type contact binaries from Leung and Schneider (1979), we find that this system has just evolved-off the Terminal Age Main Sequence. Moreover, placing RY Scuti in the [log P, log M1/M2] diagram for O+O and WR+OB binaries of Massey (1981) we find that it is in an evolutionary phase just preceeding a WR+OB stage.

V388 Cygni: a binary system in the rapid phase of mass exchange?

Using the Wilson & Devinney approach, we analyse the only existing, as far as we know, photoelectric lightcurve of V388 Cyg (P = 0.859 day, Sp = A3). The absence of any spectroscopic indication of the mass ratio was overcome by the computation of a grid of solutions with many values of q = ℳc/ℳh which fixed a well-determined range of possible q's. For every possible value of q, the primary (more massive and hotter) component fills its lobe, while the secondary, although detached, is not far from contact. It is suggested that V388 Cyg is a semi-detached system in the rapid phase of case A mass exchange, like SZ Psc and BF Aur. This possibility is discussed, taking into account the observed period variations as well. Although only spectroscopic information may solve the problem of the evolutionary stage of V388 Cyg, we find that the inferred configuration is at least very probable and that no evidence exists against it.

A lower bound to the rate of mass loss from a circular binary system evolving via emission of gravitational waves and Roche lobe overflow of degenerate component

In a spin-conserved circular binary system which is emitting gravitational waves and losing mass by overflow of the Roche lobe of the less massive and degenerate component, we find that, if the mass exchange occurs on a time scale longer than the thermal time scale, then during the mass exchange phase the orbital period must increase, implying the existence of lower limits both to the rate of mass loss from the system and from the degenerate component and an upper limit to the angular momentum loss rate.

The influence of mass loss on the evolution of binaries

Mass loss can affect the evolution of binaries in various ways, during different stages of the evolution.1. For massive stars stellar wind mass loss will change the masses of the components during their main sequence evolution.2. During the Roche lobe overflow phase (or tidal interaction phase) matter can leave the system.3. For low mass stars matter can leave the system during the mass exchange phase and can be stored in envelopes, disks or rings.4. Sufficiently massive stars(>8-15M0) undergo at the end of their life a supernova explosion, where most of the matter is blown away and a compact object, a neutron star or a black hole can be left.5. For intermediate stars one of the components can evolve into a degenerate He or CO dwarf; a reverse mass transfer can dump matter on this degenerate dwarf. If the conditions are favorable the white dwarf can explode with loss of matter, and a neutron star can be the result.6. The chemical abundances in the outer layers change.

Formation of Luminous Contact Binaries by Rapid Accretion onto White Dwarfs

During the evolution of a close binary system, there is a phase of mass exchange between its component stars. When the mass-losing star is an evolved star off the main sequence, the mass exchange is very rapid. Its rate may well exceed the critical accretion rate corresponding to the Eddington limit of the mass accreting star. Effects of such a rapid accretion were studied for the mass-accreting main-sequence stars (Flannery and Ulrich 1977; Kippenhahn and Mayer-Hofmeister 1977; Neo et al. 1977). The mass-accreting stars become quickly overluminous and their radii increase even to a red-giant size as mass is accumulated. Then the both component stars will fill their Roche lobes to form a contact binary system.

On the Existence of Double-Contact Binaries

We present observational evidence relevant to the existence of binaries of the morphological type proposed by Wilson (1979), called double-contact binaries. Consider normal evolution with Roche lobe overflow, in the rapid phase of mass exchange. It is well known that such mass-exchange spins-up the accreting star (most of whose mass is transferred material by the end of the rapid phase). Many examples are known (Table 1, for example) of Algol-types with fast-spinning primaries (original secondaries). Suppose the accreting star has been spun-up so much that it cannot accept further high-angular momentum material. It will then be filling its limiting rotational lobe (but not its Roche lobe). The secondary (original primary) will still be filling its Roche lobe. The overall result is that both stars fill their limiting lobes accurately, but are not in contact with one another. Since it is common to speak of “the contact component” of a semi-detached system - meaning the one in contact with its limiting lobe - it seems natural to call members of the new morphological type “double-contact” systems. Of course, there must be a repository for the mass lost by the Roche lobe filling and rotational lobe filling components. This would naturally be the circumstellar disks around the primary components of such systems as β Lyr and V356 Sgr (Wilson and Caldwell, 1978). The end of the double-contact phase would occur when the rate of transfer of rotational angular momentum back into the orbit, by tidal braking, begins to exceed the rate of transfer of angular momentum the opposite way by mass exchange.

Evolution of a (1+0.75) solar mass close binary system with mass exchange and gravitational radiation losses.

The evolution of two low-mass close binary systems is followed numerically through the phase of mass exchange, both with and without the inclusion of gravitational radiation losses. In both systems, the primary, mass-losing components are initially homogeneous zero-age main-sequence stars of Populations I and II, respectively, whereas the secondary components are just assumed to be compact (i.e., white dwarfs, neutron stars, or black holes). It is found that gravitational radiation, via orbital angular-momentum losses, markedly influences the evolution of the system: (a) mass loss starts much earlier; and (b) the system spends a much longer time in the mass-exchange phase with, therefore, a smaller mass-transfer rate when compared with the case where gravitational radiation is neglected. The implications for observations are briefly discussed. In particular, these systems with white dwarfs as secondaries would constitute realistic models for cataclysmic variables, whereas if the compact component is a neutron star or black hole, the systems would be producing X-rays (L/sub x/approximately-less-than10/sup 36/ ergs s/sup -1/) for about 10/sup 10/ years and would constitute low-intensity globular cluster X-ray sources.